Art-known polysiloxanes having hydrogen atoms directly bonded to silicon atoms in the polysiloxanes, i.e. hydrogenpolysiloxanes, include silanol-terminated polydihydrogensiloxane powders and polydihydrogensiloxane powders obtained by dimethylsilylating their terminals (see Japanese Unexamined Patent Application Publication [herein referred to as “JP Kokai”] Sho 59-84920 and JP Kokai Sho 60-42426), solvent-soluble silicone resins comprising silanol-terminated polydihydrogensiloxanes or silyl-terminated polydihydrogensiloxanes (see JP Kokai Sho 60-86018), low polymerized cyclic dihydrogenpolysiloxanes (see Inorg. Chem. 1983, 22, 2163-2167), hydrogensilsesquioxane resins (see U.S. Pat. No. 3,615,272), perhydrosiloxane copolymers of the chemical formula [H2SiO]x[HSiO3/2]y (where x and y represent mole fractions, 0.01≦x≦0.1, 0.9≦y≦0.99, and x+y=1) with a molecular weight ranging in value from Mn=300 to Mw=500,000 (see Japanese Examined Patent Application Publication [herein referred to as “JP Kokoku”] Hei 7-86142), silicon hydride resins represented by the structural formula (SiO2)x(RSiO3/2)y(R2SiO)z (in the above structural formula, R stands for hydrogen or a hydrocarbon, with at least 20% being hydrogen, y is a mole fraction between 0.05 and not more than 1, and x and z are mole fractions between 0 (excluding 0) and 0.95) (see Japanese Patent No. 3298990) and hydridosiloxane resins represented by the formula [H0.5˜1.0SiO1.5˜1.8]p and formula [HSiO1.5]n[SiO2]w (where p is an integer in the range of about 8 to about 5000, the sum of n and w is an integer in the range of about 8 to about 5,000, the sum of n and m is an integer in the range of about 8 to about 5,000, . . . ) (see Japanese Publication of Translation of PCT Application in Foreign Language [herein referred to as “JP Kohyo”] 2000-510522)
It is known that solutions of the above-mentioned polydihydrogensiloxanes produce silicon oxide films when subjected to heat treatment, etc., and that coating a substrate with a thin layer of a hydrogensilsesquioxane resin solution or silicon hydride resin solution, volatilizing the solvent, and heating to an elevated temperature converts it into a ceramic thin film containing silicon dioxide (i.e. silica) with a thickness of about 1 μm, but less than 2 μm (see JP Kokai Sho 60-86018 etc., JP Kokoku Sho 6-42477, and Japanese Patent No. 3298990).
However, problems with the above-mentioned polydihydrogensiloxanes include the fact that they are produced in powder form and cannot be used for coating unless dissolved in solvents, that silanol-terminated polydihydrogensiloxanes are unstable, and that polydihydrogensiloxanes with dimethylsilylated terminals have decreased inorganicity and heat resistance (see JP Kokai Sho 59-84920). The inventors herein noticed that due to the fact that the degree of polymerization of the above-mentioned low polymerized cyclic dihydrogenpolysiloxanes ranges from 4 to 23 and its main ingredient has a degree of polymerization ranging from 4 to 9, most of it volatilizes in the process of curing and cured products of the prescribed size are not obtained. Another problem is that hydrogensilsesquioxane resins, silicon hydride resins, and the above-mentioned perhydrosiloxane copolymers are solid at normal temperature and cannot be used for coating thin layers unless dissolved in organic solvents (see JP Kokoku Hei 6-42477, Japanese Patent No. 3298990, and JP Kokoku Hei 7-86142). Moreover, the inventors herein noticed that if the thickness of films obtained by coating the solutions and volatilizing the solvents reaches 2 μm or more, cracks appear in the coating films, and, even if cracks do not appear at such time, cracking starts when they are subjected to heat treatment etc. in order to convert them to silica. In addition, the inventors herein noticed that cast molding etc. is impossible and millimeter-order films, sheets, slabs, and blocks cannot be molded because the above-mentioned perhydrosiloxane copolymers, hydrogensilsesquioxane resins, and hydridosiloxane resins are solid at normal temperature and do not melt even when heated.
In addition, another problem is that the above-mentioned polydihydrogensiloxanes, which are produced by the hydrolysis/condensation of dialkoxysilanes, i.e. H2Si(OR)2, are obtained only in the form of powders that can be dissolved in solvents but do not melt when heated.
The above-mentioned hydrogensilsesquioxane resins and silicon hydride resins are produced by the “scarce water” hydrolysis method described in the Specification of U.S. Pat. No. 3,615,272, i.e., by a method, in which hydrogentrichlorosilane is hydrolyzed in benzenesulfonic acid hydrate-based hydrolysis medium and the resulting resin is then washed with water or an aqueous solution of sulfuric acid, or, more specifically, by a method, in which a benzene solution of trichlorosilane is subjected to hydrolysis/condensation via dropwise addition to a mixture of concentrated sulfuric acid, fuming sulfuric acid, and benzene, and the resulting resin is washed with water or an aqueous solution of sulfuric acid (see paragraphs [0012] and [0013] in Japanese Patent No. 3298990).
The above-mentioned perhydrosiloxane copolymers of the chemical formula [H2SiO]x[HSiO3/2]y (where x and y represent mole fractions, 0.01≦x≦0.1, 0.9≦y≦0.99, and x+y=1) with a molecular weight ranging in value from Mn=300 to Mw=500,000, i.e hydrogensiloxane copolymers, are produced by a process involving (1) preparing a hydrolysis medium containing an arylsulfonic acid hydrate, (2) adding HSiX3 and H2SiX2 (wherein X is a hydrolysable group such as Cl or an alkoxy group) to the hydrolysis medium under agitation, (3) facilitating hydrolysis of the HSiX3 and H2SiX2 in the hydrolysis medium to form the copolymer, (4) settling the hydrolysis medium and copolymer into immiscible layers comprising an acid layer and an organic layer (where the organic layer contains the copolymer), and (5) separating the organic layer from the acid layer (see paragraphs from 0011 to 0031 of JP Kokoku Hei 7-86142). In Example 1, notwithstanding co-hydrolysis/condensation of 8.5 g (0.08 mol) dichlorosilane, i.e. H2SiCl2, and 9.4 g (0.07 mol) trichlorosilane, i.e. HSiCl3, the produced hydrogensiloxane copolymer has a chemical formula of (H2SiO)1(HSiO3/2)19, in other words, it has an average siloxane unit formula of (H2SiO)0.05(HSiO3/2)0.95, which indicates a considerable decrease in the number of H2SiO units and a considerable increase in the number of HSiO3/2 units. The rate of introduction of H2SiO units was poor, and in fact, it was possible to produce only hydrogensiloxane copolymers having 5 mol % or less of H2SiO units. As a result, it has been impossible to significantly improve the physical properties of hydrogensilsesquioxanes which, similarly to hydrogensilsesquioxane resins, are solid at normal temperature. For this reason, they are typically spin-coated, spray-coated, etc., in the form of solutions in hydrocarbon solvents (for example, toluene). In other words, producing thin film coatings is a problem unless they are dissolved in hydrocarbon solvents. Moreover, the inventors herein noticed that if the thickness of films obtained by dissolving them in hydrocarbon solvents, applying the solution in the form of a thin film coating and volatilizing the solvent reaches 2 μm or more, cracks appear in the coating film, and, even if cracks do not appear at such time, cracking starts when the film is heat treated etc. in order to convert it to a silica coating. In addition, the inventors herein noticed that cast molding etc. is impossible and films, sheets, slabs, and blocks with a thickness in the millimeter order cannot be molded because the products, i.e. hydrogensiloxane copolymers, are solid at normal temperature and do not melt even when heated.
JP Kohyo 2000-510522 discloses a process for preparing hydridosiloxane resins or organohydridosiloxane resins comprising the steps of (a) contacting a silane monomer of the general formula R1SiX3 (where X is a halogen or OR2, and R1 and R2 are independently selected from the group comprising H, alkyl, and aryl groups) with a phase transfer catalyst, i.e. a quaternary ammonium salt, in the presence of a reaction mixture comprising a nonpolar solvent and a polar solvent under conditions effective to catalytically convert said silane monomer into a hydridosiloxane resin or an organohydridosiloxane resin, and (b) recovering the hydridosiloxane resin or organohydridosiloxane resin produced above. The product is exemplified by resins represented by the formula [H0.5˜1.0SiO1.5˜1.8]p and by the formula [HSiO1.5]n[SiO2]w (where p is an integer in the range of about 8 to about 5000, the sum of n and w is an integer in the range of about 8 to about 5000, and the sum of n and m is an integer in the range of about 8 to about 5000, . . . ), but only the product of hydrolysis/condensation of trichlorosilane (a hydrogensilsesquioxane resin) is mentioned in the Working Examples. Hydridosiloxane resins represented by the formula [H0.5˜1.0SiO1.5˜1.8]p and by the formula [HSiO1.5]n[SiO2]w have a smaller ratio of H to Si than the hydrogensilsesquioxane resins, in other words, they are closer to silica [SiO2] and thus are more solid at normal temperature, harder to melt when heated, harder to dissolve in solvents, and are more difficult to use for thin film coatings.
JP Kokai 2001-2785 describes a process for producing a silicone resin comprising the steps of (A) subjecting at least one chlorosilane described by the formula RxSiCl4-x (where X=0 or 1 and R=hydrogen or monovalent hydrocarbon group) to hydrolysis/condensation by adding it to a two-phase mixture comprising a nonpolar organic solvent and an aqueous phase containing a surface active compound selected from the group comprising 0 wt. % to 43 wt. % hydrochloric acid, alkylsulfonic acid hydrate, alkali metal salt of alkylsulfonic acid, arylsulfonic acid hydrate, and alkali metal salt of arylsulphonic acid, and (B) separating the two-phase mixture into an aqueous phase and an organic phase comprising the silicone resin and contacting said organic phase with a neutralizing agent; however, as far as pure hydrogenpolysiloxanes are concerned, only hydrogensilsesquioxane resins of the formula (HSiO3/2)m are described.
Incidentally, besides quartz glass, hydrogensilsesquioxane resins are the only polymeric materials possessing excellent transmittance in the vacuum UV region at 170 nm or higher to near infrared region up to 1700 nm. However, what the above-mentioned prior-art documents describe is nothing more than coating an extremely thin layer of a hydrogensilsesquioxane resin solution on a substrate, volatilizing the solvent, and then heating it to an elevated temperature to form a ceramic silica layer with a thickness of around 1 μm, but less than 2 μm. Accordingly, the processes of the above-mentioned prior-art documents are incapable of forming ceramic silica layers with a thickness of 2 μm or more on optical members made of quartz glass etc. They are, of course, even more inadequate for producing ceramic silica with a thickness in the millimeter order. Accordingly, they are unsuitable for producing optical elements that are required to possess inner structural uniformity and thickness permitting wide application in various UV light sources, primarily excimer lasers and the like.